Power quality control for grid connected Photovoltaic system with Neutral Point Converter

Abstract

The problem of interfacing a Photovoltaic Generator (PVG) with an electrical three phase grid is addressed in the presence of nonlinear loads. The connection of the PVG to the power grid is done in two stages with both a DC/DC converter and DC/AC multilevel inverter. This latter is based on a Neutral Point Converter (NPC) operating simultaneously as an interfacing system and as a shunt active power filter. The main objectives of the proposed control system are threefold: (i) transferring the optimal power from PVG to the power grid; (ii) cancelling the load current harmonics and compensating the reactive power absorbed by nonlinear loads; (iii) balancing the capacitor voltages and regulating the DC bus voltage. To this end, two closed loops of nonlinear control are developed on the basis of the averaged PV system model. The control system includes firstly an AC current controller, designed with the Backstepping control technique, and using p-q Theory for the computation of these reference signals. Secondly, a DC bus voltage controller is designed using the sliding mode control technique. Simulations show that the proposed control system achieved the listed objectives under a widely varying load and solar irradiance levels.